This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. Prior SAXS results with control proteins of known structure and several gp140 trimer forms show that SAXS is a tractable method of analysis in our hands. All samples can be prepared on short notice for SAXS data collection and are currently in routine production to support other studies. Protein monodispersity is monitored by SEC with simultaneous static/dynamic light scattering as well as quantitative/qualitative Ab binding assays in a variety of formats. We expect to obtain overall shape and size information of different gp140-NAb complexes, generate models for the functional form of envelope spike if possible, and verify by cryo-EM structures of Env spikes and crystal structures of gp120 and gp41.